Radiation diffractive materials based on crystalline colloidal arrays have been used for a variety of purposes. A crystalline colloidal array (CCA) is a three-dimensional ordered array of mono-dispersed colloidal particles. The particles are typically composed of a polymer latex such as polystyrene or an inorganic material, such as silica.
Such colloidal dispersions of particles can form crystalline structures having lattice spacings that are comparable to the wavelength of ultraviolet, visible or infrared radiation. These crystalline structures have been used for filtering narrow bands of selected wavelengths from a broad spectrum of incident radiation, while permitting the transmission of adjacent wavelengths of radiation. Prior devices have been created by dispersing particles in a liquid medium, whereby the particles self-align into an ordered array. The particles are fused together by mutual polymerization or by introducing a solvent that swells and fuses the particles together.
In other uses of CCAs, an ordered array is fixed in a matrix and may be used as colorants when the fixed array diffracts radiation in the visible spectrum. Alternatively, CCAs are fabricated to diffract radiation for use as optical filters, optical switches and optical limiters. While these CCAs use constant interparticle spacing, a CCA may function as a sensor when the interparticle spacing varies in response to stimuli.
Recently, such sensors have been produced from hydrogels containing a CCA polymerized within the hydrogel. The polymers of the hydrogel surrounding the CCA change conformation in response to a specific external stimulus. For example, the volume of the hydrogel can change in response to stimuli, including the presence of chemicals, such as metal ions in solution and organic molecules, such as glucose, making the devices useful for chemical analysis. In hydrogel-based devices, mono-dispersed highly charged colloidal particles are dispersed in a low-ionic strength liquid media. The particles self-assemble into a CCA due to their electrostatic charges. These ordered structures diffract radiation according to Bragg's law, wherein the radiation meeting the Bragg conditions are reflected while adjacent spectral regions that do not meet the Bragg conditions are transmitted through the device.
An ordered periodic array of particles that diffracts radiation according to Bragg's law satisfies the equation:mλ=2nd sin θwhere m is an integer, λ is the wavelength of reflected radiation, and n is the effective refractive index of the array, d is the distance between the layers of particles, and θ is the angle that the reflected radiation makes with the plane of a layer of particles. Incident radiation is partly reflected at an uppermost layer of particles in the array at angle θ to the plane of the first layer and is partially transmitted to underlying layers of the particles. While some absorption incident radiation occurs as well, a portion of the transmitted radiation is partially reflected at the second layer of particles in the array at angle θ and partially transmitted to the underlying layers of particles. This feature of partial reflection at angle θ and partial transmission to the underlying layers of particles continues through the thickness of the array. The wavelength (λ) of diffracted radiation can be controlled by the dimension d, which may be the distance between the planes of the centers of the particles in each layer. Initially, the diffracted wavelength λ is proportional to the particle diameter for an array of packed particles. However, when distance (d) between layers of particles in a periodic ordered array increases, the wavelength of diffracted radiation also increases. Sensor devices that increase the interspatial volume within the device in response to a specific chemical species increase the interspatial distance between layers of particles, thereby altering the wavelength of diffracted radiation.
In a hydrogel-based CCA, when the volume of the hydrogel changes, the diffraction wavelength of the CCA changes. Such CCA devices that are based on hydrogels typically contain a large percentage of water, such as about 30% by volume. These hydrogel-based CCAs are fragile and have a propensity for significant changes in their optical performance when the water content of the CCA changes.
To overcome these drawbacks of hydrogel-based CCAs, one approach has been to prepare a hydrogel-based CCA, dehydrate the hydrogel matrix surrounding the CCA and then back fill the array with a polymerizable monomer. The monomer is polymerized to produce an essentially water-free polymerized crystalline colloidal array. These arrays respond to certain environmental stimuli, such as compressive stress (thereby altering the lattice spacing) to alter the diffracted wavelength of the CCA.
However, these prior systems of hydrogel-based CCAs have significant production and handling drawbacks. A need exists for a more robust CCA, which exhibits radiation diffracting properties in response to applied chemical stimuli and the like and which substantially returns to its initial optical characteristics upon removal of the stimulus.